For optical fibers, the refractive index profile is generally qualified in relation to a graph, plotting refractive index as a function of optical fiber radius. Conventionally, the distance r to the center of the optical fiber is shown along the abscissa (i.e., the x axis), and the difference between the refractive index at radius r and the refractive index of the outer optical cladding of the optical fiber is shown along the ordinate axis (i.e., the y axis). The outer optical cladding has a constant refractive index and usually consists of pure silica. The outer optical cladding, however, may also contain one or more dopants. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, or a triangle. These curves are generally examples of the theoretical or set profile of the optical fiber. The manufacturing stresses of the optical fiber may lead to a slightly different profile.
An optical fiber typically includes an optical core, whose function is to transmit and possibly to amplify an optical signal, and an optical cladding, whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and the outer cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber is divided into a fundamental mode (i.e., dominant mode) guided in the core and into secondary modes (i.e., cladding modes) guided over a certain distance in the core-cladding assembly.
Conventionally, step-index fibers, also called single-mode fibers or (SMF), are used as a line fiber for transmission systems employing optical fibers. These optical fibers exhibit a chromatic dispersion and a chromatic dispersion slope meeting specific telecommunications standards, as well as normalized values for the effective area and the cutoff wavelength.
Typically, for terrestrial transmission systems, standard single-mode fibers (SSMF) are used, which have a positive dispersion (D) and a positive dispersion slope (P), an effective area (S) of about 80 μm2 and an attenuation of about 0.19 dB/km (measured at a wavelength of 1550 nm). Submarine transmission systems with repeaters typically use hybrid transmission lines with optical fibers having a positive dispersion, a large effective area (about 100-110 μm2), and a low attenuation (0.17-0.19 dB/km measured at a wavelength of 1550 nm), and optical fibers with negative dispersion.
Submarine transmission systems without repeaters typically use transmission lines that include combinations of optical fibers having a positive dispersion and an effective area of between 80 and 110 μm2.
As known by those having ordinary skill in the art, an increase in the effective area of a transmission optical fiber contributes to the reduction of non-linear effects in the optical fiber. A transmission optical fiber having an enlarged effective area facilitates transmission over a longer distance and/or an increase in the functional bands of the transmission system. To increase the effective area of a transmission optical fiber, optical fiber profiles with an enlarged and flattened central core as compared to a conventional SSMF were proposed. Such a change in the shape of the central core of the optical fiber, however, leads to an increase in the bending and microbending losses and to an increase of the effective cutoff wavelength. The effective cutoff wavelength is conventionally measured as the wavelength at which the optical signal is single mode after propagation over two meters of optical fiber. This is defined by subcommittee 86A of the International Electrotechnical Commission under standard IEC 60793-1-44.
U.S. Pat. No. 6,658,190, which is hereby incorporated by reference in its entirety, describes a transmission optical fiber with an enlarged effective area of more than 110 μm2. This optical fiber has a wide central core (11.5 μm-23.0 μm), that is 1.5× to 2× that of a SSMF and a configuration with a constant or slightly depressed cladding. To compensate for the increase in bending losses caused by an increase in the effective area, this patent proposes to increase the diameter of the optical fiber. See FIG. 29 of U.S. Pat. No. 6,658,190. Such an increase in the optical fiber diameter, however, involves costs and, in addition, causes cabling problems as the resulting fiber may be incompatible with other optical fibers. This patent further discloses that the cutoff wavelength decreases with the length of the considered optical fiber. (See FIG. 5 of U.S. Pat. No. 6,658,190.) Finally, this patent notes that the optical fiber reaches single-mode operation after one kilometer of transmission. Such a measurement of the cutoff wavelength, however, does not comply with the aforementioned normalized measurements.
The publication of Masao Tsukitani et al. entitled “Ultra Low Nonlinearity Pure-Silica-Core Fiber with an Effective Area of 211 μm2 and Transmission Loss of 0.159 dB/km,” M3.3.2, ECOC 2002, (Sep. 9, 2002), describes an optical fiber with a refractive index profile configuration having a wide and slightly depressed cladding adjacent to a central core. Such an optical fiber has an effective area of 211 μm2 and low attenuation. To limit the bending losses, however, the diameter of the optical fiber was increased to 170 μm, (versus 125 μm for a SSMF). This leads to significant manufacturing costs and problems of incompatibility with other optical fibers.
Optical fiber configurations for increasing the effective area were proposed in the publication of Kazumasa Ohsono et al. entitled “The Study of Ultra Large Effective Area Fiber & Mating Dispersion Slope Compensating Fiber for Dispersion Flattened Hybrid Optical Fiber DWDM Link,” IWCS 2002, pp. 483-487, (Nov. 18, 2002), and in the publication of Kazuhiko Aikawa et al. entitled “Single-Mode Optical Fiber with Effective Core Area larger than 160 μm2,” ECOC 1999, pages 1-302, (Sep. 26, 1999).
Moreover, U.S. Pat. No. 6,665,482, which is hereby incorporated by reference in its entirety, proposes a pedestal refractive index profile for achieving an effective area of more than 90 μm2. In its examples, however, the values of the effective area are less than 110 μm2.
U.S. Pat. No. 5,781,684, which is hereby incorporated by reference in its entirety, describes a coaxial optical fiber having a large effective area for a dispersion-shifted fiber, also called Non-Zero Dispersion Shifted Fiber (NZDSF). This optical fiber has a cutoff wavelength that is too high for maintaining single-mode in the C+ band (1530 nm-1570 nm), and a mode field diameter that is too small (less than 11 μm at 1550 nm).
U.S. Patent Application Publication No. 2005/0244120, which is hereby incorporated by reference in its entirety, describes an optical fiber with a large effective area (>75 μm2) and a low attenuation (<0.20 dB/km at 1550 nm). The optical fiber described in this publication has a refractive index profile with a central core, an intermediate cladding, and a depressed cladding. The depressed cladding, however, is too wide (7 μm to 7.4 μm) or not sufficiently buried (−0.1 percent) to achieve the combination of a large effective area and a low effective cutoff wavelength.
U.S. Pat. No. 6,483,975, which is hereby incorporated by reference in its entirety, describes an optical fiber with a large effective area (>100.0 μm2) and a positive chromatic dispersion (>20 ps/(nm·km)). Several optical fiber refractive index profiles are described in this patent, including a profile with a central core, an intermediate cladding, and a depressed cladding. See FIGS. 5a-5b of U.S. Pat. No. 6,483,975. The depressed cladding, however, is too wide (width r3−r2 of between 15 μm and 19 μm) and too close to the central core (width of intermediate cladding r2−r1 of 2-4 μm) to achieve the optical characteristics desired by the present invention.
U.S. Pat. No. 4,852,968, which is hereby incorporated by reference in its entirety, describes an optical fiber having a refractive index profile with a depressed cladding. This patent aims to improve certain optical parameters of the optical fiber (e.g., the dispersion, confinement, and bending loss parameters) by the presence of a depressed cladding. This patent, however, does not mention the impact on the effective cutoff wavelength or on the effective area. Only a mode field diameter of 9.38 μm is mentioned, but this would lead to an effective area of less than 80 μm2.
European Application No. 1,477,831 and its counterpart U.S. Pat. No. 6,904,218, which is hereby incorporated by reference in its entirety, describe an optical fiber with a large effective area (>80 μm2) and a cutoff wavelength limited to 1310 nm. Several optical fiber profiles are described in these patent documents and, notably, a refractive index profile with a central core, an intermediate cladding, and a depressed cladding. See FIG. 8 of European Application No. 1,477,831. The depressed cladding, however, is too wide (about 15 μm) to achieve exceptional optical characteristics. Moreover, the outer diameter of the depressed cladding is large (about 33 μm), which involves significant manufacturing costs.
Therefore, there exists a need for a transmission of optical fiber that has an enlarged effective area of more than 120 μm2 without degrading other optical fiber parameters (e.g., losses and dispersion), and that has effective cutoff wavelength of less than 1600 nm.